Cardiovascular risk and mortality is uniquely elevated among midlife women due to factors such as menopause and gender differences in cardiovascular event symptom presentation. Physical activity (PA) is critical to reducing risk in midlife women; however, this group confronts distinctive psychosocial barriers to PA that rarely are addressed by existing interventions. Current evidence and the candidate?s pilot data indicate that these barriers are negative mood, body satisfaction, and social comparison (i.e., evaluating one?s PA relative to that of peers). Mobile health tools such as smartphone applications have the ability to intervene on these barriers in real time, but developing effective tools requires a sophisticated understanding of (1) when PA occurs and determinants of PA for particular individuals in their natural environments, and (2) the ability to optimize this information for mobile delivery. The proposed K23 training program will address these needs by focusing on the following training goals: ambulatory assessment of PA and its psychosocial determinants, advanced statistical methods, individual tailoring of PA programs (with an emphasis on mobile intervention development), women?s health, and cardiovascular physiology. Training will include apprenticeships and tutorials with experts in each of these topics, as well as coursework in intensive ambulatory assessment, mobile application design, and physiology. To support this plan, the proposed research will investigate PA and its determinants among midlife female primary care patients with cardiovascular risk markers. Aim 1 of this research is to examine relations between real-time psychosocial experiences (i.e., mood, body satisfaction, social comparison) and PA among at-risk midlife women (n=100, age 40-60; 27>BMI<50 with one additional cardiovascular risk marker). This will allow for further optimization of a mobile health tool tailored for midlife women. Aim 2 of this research is to refine and pilot test a novel mobile PA application tailored for at-risk midlife women (n=30, age 40-60; 27>BMI<50 with one additional CVD risk marker). Achieving these project aims will produce multiple peer-reviewed manuscripts and a strong R01 proposal focused on testing a well-informed, tailored mobile intervention for midlife women at risk for developing CVD. This research builds on the candidate?s training to date and introduces an innovative way to understand and promote PA in midlife women. The goals of this K23 program are directly responsive to NHLBI?s strategic plan, as they will allow for improved primary prevention through increased understanding of determinants of disease risk, and the proposed program will contribute to improving interventions tailored to patient needs and barriers. The excellent training, protected time, and project resources of the proposed K23 program will prepare the candidate to become a leading patient-oriented researcher with expertise in PA promotion for cardiovascular risk reduction among midlife women.